Traditionally, trenches in a semiconductor substrate are oxidized and refilled with polysilicon material or entirely filled with deposited oxide. The trench refill process completely fills the trenches and leaves no voids. Voids on the surface of the polysilicon, when oxidized, cause stress within the substrate which subsequently generate yield reducing defects. Manufacturers of semiconductor substrates, therefore, strive to great lengths to avoid stress within the substrate. For example in U.S. Pat. No. 5,448,102 (Gaul et al.), Gaul et al. disclose a process to avoid and reduce stress in a semiconductor substrate that is induced by filling trenches.
Manufacturers also strive to remove or getter impurities. Often heavy elements, such as iron, contaminate device wafers. Those impurities can be attracted to gettering sites that are spaced from the front surface of the wafer where devices are formed. Typical gettering mechanisms include abrading the back surface of the wafer or coating the back surface with a gettering agent, such as polysilicon. Oxygen can be a gettering agent. Thermal processing is used to form zones in the front surface that are denuded of oxygen. Impurities are captured in the remaining oxygen sites that are in the wafer but below the front surface and beneath the denuded zones.